1. Field of the Invention
The present invention relates to a silica glass composition, and more particularly, to a composition for use in preparing silica glass by a sol-gel process.
2. Description of the Related Art
Generally, silica glass is transparent, chemically inert and has useful characteristics including excellent thermal stability, strength and a low thermal expansion coefficient. These characteristics allow silica glass to be extremely useful for optical devices such as optical fibers or optical lenses.
An optical fiber is basically comprised of a core and a cladding having a refractive index different from that of the core so that total reflection of light occurs at the core. In connection with the total reflection of light, the refractive index of the cladding is usually about 1% lower than that of the core. A core having a refractive index of 1.47 and a cladding having a refractive index of 1.46 are generally used.
To manufacture an optical fiber, an optical fiber preform comprised of a core rod and an overcladding tube surrounding the core rod is first fabricated. Then, the optical fiber preform is heat-treated and elongated to form the optical fiber. The manufacture of the overcladding tube can require special silica glass manufacturing processes.
Some examples of processes for making silica glass of the contemporary art are shown, for example, in the following U.S. Patents. U.S. Pat. No. 5,169,421 to Yagi et al., entitled Method Of Manufacturing Silica Glass Optical Waveguide Preform, discusses an extrusion method for forming a cladding. This method does not use a sol-gel process, but rather uses a kneadable plastic composition of silica with a higher ratio of silica to water than found in a gel. This composition also contains an additive referred to by Yagi et al. as a binder, which can be methylcellulose or another organic substance. This patent discusses a composition with three parts of methylcellulose to 100 parts silica. U.S. Pat. No. 5,314,520 to Yagi et al., entitled Method for Manufacturing Optical Fiber Preform, also discusses an extrusion method and a composition with three parts methylcellulose to 100 parts silica. U.S. Pat. No. 4,801,318, to Toki et al., entitled Silica Glass Formation Process, discusses a sol-gel process for making silica glass involving hydrolyzing a silicon alkoxide solution. U.S. Pat. No. 5,565,014, to Fleming, entitled Process Of Manufacturing A Vitreous Silica Product Including Hydorthermally Drying A Collordal Sol-Gel, describes a sol-gel process involving the addition of polyethyloxazoline to the sol before gelling. U.S. Pat. No. 5,240,488, to Chandross et al, Manufacture Of Vitreous Silica Product Via A Sol-Gel Process Using A Polymer Additive, discusses a sol-gel process involving an additive which is an organic polymer and which is sometimes referred to as a binder. Characteristics of potential additives are discussed, but the only materials specifically discussed as additives are polyethyloxazoline, polymethyloxazoline, polyacrylamide, and cationic polymers containing [--CH.sub.2 --N+(CH.sub.3).sub.2 --CH.sub.2 --] units in the backbone.
One method for manufacturing an overcladding tube formed of silica glass using a sol-gel process will now be briefly described. First, silica particles are mixed with deionized water and a dispersant. Then, a binder and a plasticizer are added to the mixture and then mixed sufficiently to form a sol. The sol is subjected to an aging-treatment for a predetermined time. Air bubbles are removed from the aging-treated sol, a gelling agent is added to the sol which is then poured into a mold.
When gelation is completed, the resultant gel is removed from the mold and then dried. Thereafter, the dried gel is heat-treated to remove organic substances in the gel. Subsequently, dehydroxylation and sintering processes are performed on the resulting gel, thereby completing an overcladding tube formed of silica glass.
In the above-described manufacturing process, polyethyloxazoline is usually used as the binder. However, because this compound is hazardous to humans and expensive, there is an increasing demand and a need for a material to be a substitute for polyethyloxazoline. Moreover, a wet gel prepared using polyethyloxazoline is not strong enough to be easily handled in subsequent processing. More strength would be highly desirable in the wet gel.
Finding a replacement for polyethyloxazoline is not a trivial task. The additive, or binder, for the silica glass composition must satisfy the following requirements. First, the binder must be highly soluble in a sol dispersion medium. Second, the binder should be homogeneously dispersed between particles, but be present in a small amount. If the binder is used excessively, too many pores are generated in the silica glass, which is not desirable.
Based on my observation of the art, I have discovered that what is needed is a silica sol-gel composition which yields improved strength in the gelled sol. Moreover, this composition should not contain a toxic polymer such as polyethyloxazoline and should be less expensive than other compositions.